"Killer ape" scene in Stanley Kubrick's 1968 film 2001: A Space Odyssey has no basis in fact.

The Deep Roots Theory addresses not just violent human aggression in general but a particular manifestation of it, involving attacks by one group against another. Deep Rooters often contend that--as warlike as we are today--we were much more warlike before the advent of civilization.

Pinker claims in his bestseller Better Angels of Our Nature that "chronic raiding and feuding characterize life in a state of nature." In The Social Conquest of the Earth, Wilson calls warfare "humanity's hereditary curse." The Deep Roots Theory has become extraordinarily popular, especially considering that the evidence for it is extraordinarily flimsy (see "Further Reading" below).

A study published today in Science, "Lethal Aggression in Mobile Forager Bands and Implications for the Origins of War," provides more counter-evidence to the Deep Roots Theory. The study's authors, anthropologists Douglas Fry and Patrik Soderberg of Abo Akademi University in Finland, say their findings "contradict recent assertions that [mobile foragers] regularly engage in coalitionary war against other groups."

Fry and Soderberg focus on mobile forager bands, also called nomadic hunter-gatherers, because their behavior is thought to provide a window into human evolution. Our ancestors lived as wandering foragers from the emergence of the Homo genus some 2 million years ago until about 10,000 years ago, when humans began raising crops, domesticating animals and settling down into more complex, hierarchical societies.

Fry and Soderberg examine data on deadly violence within 21 mobile foraging societies observed by ethnographers. The societies include the Aranda and Tiwi of Australia; Kaska, Copper Inuit and Montagnais of North America; Botocudo of South America; !Kung, Hadza and Mbuti of Africa; and Vedda and Andamanese of South Asia.

Fry and Soderberg count a total of 148 "lethal aggression events" in the societies. The researchers distinguish between violence involving people who belong to the same group and are often related; and violence between people in different groups. They also distinguish between violence involving just one perpetrator and victim and violence involving at least two killers and two victims.

These distinctions are crucial, because war by definition is a group activity. Deep Rooters often count all forms of deadly violence, not just group violence, as evidence of their theory. (They also often count violence in societies that practice horticulture, such as the Amazonian Yanomamo, even though horticulture is a relatively recent human invention.)

Of the 21 societies examined by Fry and Soderberg, three had no observed killings of any kind, and 10 had no killings carried out by more than one perpetrator. In only six societies did ethnographers record killings that involved two or more perpetrators and two or more victims. However, a single society, the Tiwi of Australia, accounted for almost all of these group killings.

Some other points of interest: 96 percent of the killers were male. No surprise there. But some readers may be surprised that only two out of 148 killings stemmed from a fight over "resources," such as a hunting ground, water hole or fruit tree. Nine episodes of lethal aggression involved husbands killing wives; three involved "execution" of an individual in a group by other members of the group; seven involved execution of "outsiders," such as colonizers or missionaries.

Most of the killings stemmed from what Fry and Soderberg categorize as "miscellaneous personal disputes," involving jealousy, theft, insults and so on. The most common specific cause of deadly violence—involving either single or multiple perpetrators--was revenge for a previous attack.

These data corroborate a theory of warfare advanced by Margaret Mead in 1940. Noting that some simple foraging societies, such as Australian aborigines, can be warlike, Mead rejected the idea that war was a consequence of civilization. But she also dismissed the notion that war is innate--a "biological necessity," as she put it--simply by pointing out (as Fry and Soderberg do) that some societies do not engage in intergroup violence.

Mead (again like Fry and Soderberg) found no evidence for what could be called the Malthusian theory of war, which holds that war is the inevitable consequence of competition for resources.

Instead, Mead proposed that war is a cultural "invention"—in modern lingo, a meme, that can arise in any society, from the simplest to the most complex. Once it arises, war often becomes self-perpetuating, with attacks by one group provoking reprisals and pre-emptive attacks by others.

The war meme also transforms societies, militarizes them, in ways that make war more likely. The Tiwi seem to be a society that has embraced war as a way of life. So is the United States of America.

The Deep Roots Theory is insidious because it leads many people to succumb to the fatalistic notion that war is inevitable. Wrong. War is neither innate nor inevitable.

Horgan, "Worst Column Ever By Times Pundit David Brooks: 'When the Good Do Bad'": http://blogs.scientificamerican.com/cross-check/2012/05/21/worst-column-ever-by-times-pundit-david-brooks-when-the-good-do-bad/

Horgan: "Are We Doomed to Wage Wars Over Water?": http://blogs.scientificamerican.com/cross-check/2012/03/26/are-we-doomed-to-wage-wars-over-water/

Horgan, "RIP Military Historian John Keegan, Who Saw War As Product of Culture Rather than Biology": http://blogs.scientificamerican.com/cross-check/2012/08/04/rip-military-historian-john-keegan-who-saw-war-as-product-of-culture-rather-than-biology/

Every week, hockey-playing science writer John Horgan takes a puckish, provocative look at breaking science. A teacher at Stevens Institute of Technology, Horgan is the author of four books, including The End of Science, 1996, re-published with new preface 2015; and The End of War, 2012, paperback published 2014.
Follow on Twitter @Horganism

The views expressed are those of the author and are not necessarily those of Scientific American.

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